This invention relates to a quiet magnetic resonance imaging apparatus (hereinafter referred to as the "MRI apparatus"). More particularly, it relates to an improvement in a gradient magnetic field generation apparatus.
The MRI apparatus irradiates electromagnetic waves to an inspection object placed in a static magnetic field to generate a nuclear magnetic resonance phenomenon in the atomic nuclei of the inspection object and acquires images representing the physical properties of the inspection object on the basis of magnetic resonance signals generated from the inspection object. Generally, the MRI apparatus comprises magnetic field generation means for generating a static magnetic field and a gradient magnetic field, respectively, a radio frequency coil for irradiating electromagnetic waves to the inspection object or detecting nuclear magnetic resonance (NMR) signals from the inspection object and image reconstruction means for reconstructing images by using the NMR signals so detected. The gradient magnetic field is applied in superposition with the static magnetic field so as to add position information to the NMR signal, and comprises a gradient magnetic field coil positioned inside the magnetic field generated by the static magnetic field generation means and a retaining member of the coil. It is driven when a pulsatile current is caused to flow through the gradient magnetic field coil. In this case, when a pulse current is caused to flow inside the magnetic field, an electromagnetic force acts in accordance with the Fleming's left-hand rule. This electromagnetic force causes deformation of the gradient magnetic field coil and generates noise and vibration from the static magnetic field generation apparatus. Sound-proofing or silencing of this noise is preferably effected because they impart fear and offensive feeling to a patient as the inspection object.
Accordingly, the conventional MRI apparatuses employ the system which reduces the noise of the gradient magnetic field by disposing a sound-proofing member, etc., inside a decorative cover which covers the outer periphery of the apparatus, uses a vibration-proofing material for the retaining member for holding the gradient magnetic field coil, reduces the absolute value of the amplitude of vibration by utilizing damping characteristics of the vibration-proofing member, and shortens the damping time (refer to WO 88/02912).
On the other hand, an active silencing method is known as the method of reducing various kinds of noises. This method generates a sound wave having an opposite phase to that of the noise but having the same amplitude, from an additional sound source so as to cancel the noise. A variety of methods of this kind have been proposed depending on the kinds of noises, and attempts have been made to apply these methods to the MRI apparatus.
For example, JP-A-2-70195 describes a silencing method which generates a sound from an acoustic speaker so as to minimize energy of an output signal of an acoustic sensor disposed on the downstream side of a propagation path by using the output of an acoustic sensor for detecting information of sound waves from a noise source side as a reference signal, as a technology of effecting active sound cancellation inside the propagation path of the sound source.
Further, U.S. Pat. No. 5,022,272 describes a method which detects vibration of the apparatus by using a piezoelectric device, and generates vibration having an opposite phase to that of the vibration detected to offset the vibration.